Re: [Pce] Association Source in draft-ietf-pce-segment-routing-policy-cp-01

[Dhruv Dhody <[e-mail]>]

Hi Mike,

On Fri, Nov 6, 2020 at 5:08 PM Mike Koldychev (mkoldych)
<mkoldych=[e-mail]> wrote:
>
> Hi Dhruv,
>
> I don't think it's valid to dismiss race conditions in the protocol because they are "rare". If they can happen at all means that implementations need to have extra logic to handle these race conditions.
>

Doesn't this "extra" logic exist anyway, as you must make sure there
is only one SR policy association with a given set of SR Policy
parameters under normal operations.

> What is rare today in your deployment, may not be rare tomorrow in another deployment. I can think of a case where a PCC connects simultaneously to many PCEs which create many thousands of SR CPs on it. If they all send PCInit messages before the head-end replies to them with PCRpt, then you will have this "rare" race condition repeated thousands of times. Each PCE will choose different Source/ID in PCInit and PCC will have to send PCError back to them.
>

You make a good point here! What you describe is "possible"!

> Furthermore, you need logic on the PCC to choose the right Association out of many.

The logic is the first association created for a given SR policy at
the PCC and that's it!

> There are also undesirable security/privacy implications of putting PCE/NMS IP address into the Source. It means that if two PCEs are controlling different CPs of the same policy, then one of the PCEs can learn the IP address of the other by reading the Association Source of the PCRpt messages from the PCC. This is especially bad, because this Association Source has no semantic meaning in SR Policy and may be hidden in normal show commands. Furthermore, this IP address of the PCE/NMS that created the policy will be associated to the Policy even after PCE disconnects from the PCC, as long as any CP remains in that Policy.
>

If any of this is really an issue, we got to update RFC 8697! I am not
advocating for that BTW :)
Privacy of one PCE from another in an SR domain  (under same
administrative control) is quite a stretch IMHO.

BTW, what are your thoughts on the operator-configured association in
the previous email? Not viable?

> All of this can be avoided if we just allow Source/ID to be 0 in PCInit messages. Is that really such a big change?
>

No, but the WG worked on RFC 8697 to make sure all the associations
can be handled in a common way as much as possible. When deviating
from that, IMHO a higher bar should be met. The WG should ponder if it
is met here based on the scenario described above, that's all.

Thanks!
Dhruv

PS. Process comment - If the WG decides this is needed (and I am in
rough), the procedure needs to be generic and outside the SR policy
draft in a separate I-D, so that other associations can also use it.

> Thanks,
> Mike.
>
> -----Original Message-----
> From: Dhruv Dhody <[e-mail]>
> Sent: Friday, November 6, 2020 5:15 AM
> To: Mike Koldychev (mkoldych) <[e-mail]>
> Cc: Stone, Andrew (Nokia - CA/Ottawa) <[e-mail]>; Cyril Margaria <[e-mail]>; [e-mail]; [e-mail]; pce-chairs <[e-mail]>
> Subject: Re: [Pce] Association Source in draft-ietf-pce-segment-routing-policy-cp-01
>
> Hi Mike, Andrew, Cyril,
>
> Thanks for a great discussion, more inline...
>
> On Fri, Nov 6, 2020 at 7:23 AM Mike Koldychev (mkoldych) <[e-mail]> wrote:
> >
> > Hi Andrew,
> >
> > See inline with [MK]
> >
> > Hi Mike, Dhruv, Cyril:
> >
> > We do use errors on initiate messages during race conditions, for example, symbolic name uniqueness on pce-initiated vanilla LSPs. So error based protection to enforce uniqueness and protect race conditions is manageable/done today.
> >
> > [MK] The chances of symbolic-names being the same is much less than the chance of Association Sources being different. Also the symbolic-name actually has an important meaning, the Association Source has no meaning. Getting a flood of PCError messages about a field that has no meaning would be bad. If we can eliminate these error messages completely, why not do that?
> >
> >
>
> [DD] First, the flood of errors is a stretch by any means :) And I agree with Andrew about the 'initiate' case.
>
> >
> > However: would it make sense for the SRPAG definition, to be defined by the ‘first’ entity which created the candidate path? when it’s a shared construct with other entities which are now forced to re-use that value? Using a Virtual IP on the PCE(s) would certainly help, but wouldn’t work correctly with mixed use PCC/PCE init candidate paths (would anyone do that?), or different vendor PCE/clusters/different virtual IPs would add more complexity.  The common element I see is the uniqueness on PCC and the fact that SRPAG==SRPolicy. Since Association Source is ‘scoping for the association ID’, and there is no association scoping used/needed, then the value is essentially unused – therefore just a dummy value assigned by PCC?
> >
> > [MK] Yes it’s unused! I like the idea of PCC choosing it.
> >
>
> [DD] For a dynamic association, the default is for the local PCEP speaker to be the association source unless local policy says otherwise.
>
> Anyways, based on the requirement that you had in the earlier email -
>
> Mike> 1. all 3 parties: PCC, PCE1 and PCE2 agree on the same source, AND
> Mike> 2. they agree without talking to each other.
>
> One can make the SRPolicy association to be considered as an operator-configured association (i.e. association parameters configured by the operator beforehand on the PCEP peers).
>
> Hear me out, we anyway have the SR Policy configuration happening at all peers, could we say that the PCEP association parameters
> (type/id/source..) need also be set by the operator. If you are worried that it would be extra configuration, there could be rules set by the operator for filling the association parameters using SRPolicy such as Assoc-type=SR-Policy, Assoc-ID/Extended Association ID=Color, Assoc-source=headend/special value.
>
> Note that allowing SRPolicy to be both dynamic and operator-configured is also a possiblity.
>
> >
> >
> > I think there is a bit of ambiguity as mentioned (PCEP session? Router ID? ), and still run the risk that PCEP is terminating on different addresses towards different PCEs / different view of the ‘PCC address’.  Requesting the PCC to just assign the (unused?) value seems to like a way to avoid all of the above.  With that said, I could be missing other implications/usage of the Association Source field.
> >
> > [MK] Yes, requesting the PCC to assign a value would resolve this issue. But the question is what value would the PCE send in PCInit message when first creating a policy? I propose that PCE can send just 0.0.0.0 or 0::0 in PCInit messages to indicate to the PCC to pick a value. Alternatively, PCE can send any value of Association Source/ID, but the PCC will not honor it and just choose its own Association Source/ID.
> >
> >
>
> I would like us (as a WG) to explore if we can use existing mechanisms first (the very reason we have common association groupings).
> As of now, I am not sold that the use of error in a 'rare'
> race-condition is such a bad protocol design that we need to update
> RFC8697 and introduce new rules for dynamic associations, esp when other ways exist.
>
> What do others think?
>
> Thanks!
> Dhruv
>
> >
> > Cheers
> > Andrew
> >
> >
> >
> > From: Pce <[e-mail]> on behalf of "Mike Koldychev
> > (mkoldych)" <mkoldych=[e-mail]>
> > Date: Thursday, November 5, 2020 at 1:30 PM
> > To: Cyril Margaria <[e-mail]>, Dhruv Dhody
> > <[e-mail]>
> > Cc: "[e-mail]" <[e-mail]>,
> > "[e-mail]"
> > <[e-mail]>, pce-chairs
> > <[e-mail]>
> > Subject: Re: [Pce] Association Source in
> > draft-ietf-pce-segment-routing-policy-cp-01
> >
> >
> >
> > Hi Cyril,
> >
> >
> >
> > See inline with [MK]
> >
> >
> >
> > From: Cyril Margaria <[e-mail]>
> > Sent: Thursday, November 5, 2020 11:35 AM
> > To: Dhruv Dhody <[e-mail]>
> > Cc: Mike Koldychev (mkoldych) <[e-mail]>; [e-mail];
> > pce-chairs <[e-mail]>;
> > [e-mail]
> > Subject: Re: [Pce] Association Source in
> > draft-ietf-pce-segment-routing-policy-cp-01
> >
> >
> >
> >
> >
> > I have a related question: how do you define the "PCC address", PCEP session address , one router id?
> >
> > [MK] By PCC Address, I meant the IP address of the PCEP session. I believe a better approach is actually to set Association Source in PCInitiate message to 0.0.0.0 or 0::0 and let the PCC allocate the correct Source, same as how Association ID allocation is proposed in the draft.
> >
> >
> >
> >
> >
> > For the association source race condition, the race condition will result in a "Conflicting SRPAG TLV" from a PCInitiate/PCUpd, the PCE can use the correct SRPAG.
> >
> > [MK] It’s not a good protocol design to allow PCError messages to appear randomly when all the parties are following the protocol. Would really like to avoid that.
> >
> >
> >
> >
> >
> >
> >
> > On Thu, 5 Nov 2020 at 16:16, Dhruv Dhody <[e-mail]> wrote:
> >
> > Hi Mike,
> >
> > On Thu, Nov 5, 2020 at 9:34 PM Mike Koldychev (mkoldych)
> > <[e-mail]> wrote:
> > >
> > > Hi Dhruv,
> > >
> > >
> > >
> > > Perhaps we can avoid this by letting PCE send PCInitiate message with Association Source set to some reserved value, like 0. This can mean that the PCE is basically requesting the PCC to allocate an Association Source and to “own” that Association. We already do this with the Association ID. PCE sets the ID to 0 in PCInitiate and PCC chooses an Association ID and reports it back.
> > >
> > >
> >
> > The comment was applicable for association-id as well as
> > association-source! The use of 0 as association ID is being introduced
> > by your draft and not part of the base RFC 8697 and that triggered the
> > original email. Julien and I were uncomfortable with that and wanted
> > to understand what is new here for SR policy association that requires
> > a new procedure and cant be handled by RFC 8697.
> >
> > Thanks,
> > Dhruv
> >
> > >
> > > Thanks,
> > >
> > > Mike.
> > >
> > >
> > >
> > > From: Dhruv Dhody <[e-mail]>
> > > Sent: Thursday, November 5, 2020 10:43 AM
> > > To: Mike Koldychev (mkoldych) <[e-mail]>
> > > Cc: [e-mail]; [e-mail];
> > > pce-chairs <[e-mail]>
> > > Subject: Re: Association Source in
> > > draft-ietf-pce-segment-routing-policy-cp-01
> > >
> > >
> > >
> > > Hi Mike,
> > >
> > >
> > >
> > > On Thu, Nov 5, 2020 at 7:51 PM Mike Koldychev (mkoldych) <[e-mail]> wrote:
> > >
> > > Hi Dhruv,
> > >
> > >
> > >
> > > Thanks for bringing this up.
> > >
> > >
> > >
> > > By setting ASSO_SOURCE = PCC_ADDRESS, we guarantee that:
> > >
> > > all 3 parties: PCC, PCE1 and PCE2 agree on the same source, AND they
> > > agree without talking to each other.
> > >
> > >
> > >
> > > In your proposal below, if we set ASSO_SOURCE = NMS_ADDRESS, it seems that condition 1 may be fulfilled, but it requires exchange of PCRupt/PCUpd messages between the 3 entities, which violates condition 2. Please correct me if I misunderstood something. In the picture that you drew, you say that “Policy Endpoint=X” and “Association Source=X”, are you suggesting to use the policy endpoint as the ASSO_SOURCE? That would satisfy both conditions, but I’m not sure if you intended that?
> > >
> > >
> > >
> > >
> > >
> > > No, I did not!
> > >
> > >
> > >
> > >
> > >
> > > I believe condition 2 is important to satisfy, because otherwise there could be race conditions where the 3 parties have different ASSOC_SOURCE for the same policy. Consider what happens when all 3 parties try to create the same policy at the same time.
> > >
> > >
> > >
> > >
> > >
> > > The SR-Policy association is "dynamic" in nature, and we need to go by the association parameters we receive from the PCEP peer. Condition 2 of talking to each other is the very nature of a dynamic association!
> > >
> > >
> > >
> > > If the race condition is the issue to solve, we can use the SR-Policy parameters (color, endpoint, source). And make sure there is only one SR-Policy-association-group with a given set of SR-Policy parameters (and generate an error otherwise). The other PCE would learn about the association and can use it subsequently!
> > >
> > >
> > >
> > > I’m open to any proposal, but IMO we should respect the above two requirements.
> > >
> > >
> > >
> > >
> > >
> > > I feel the requirement 2 is not compatible with a dynamic association.
> > >
> > >
> > >
> > > Thanks!
> > >
> > > Dhruv
> > >
> > >
> > >
> > >
> > >
> > > Thanks,
> > >
> > > Mike.
> > >
> > >
> > >
> > > From: Dhruv Dhody <[e-mail]>
> > > Sent: Thursday, November 5, 2020 1:59 AM
> > > To: [e-mail]
> > > Cc: [e-mail]; pce-chairs <[e-mail]>
> > > Subject: Association Source in
> > > draft-ietf-pce-segment-routing-policy-cp-01
> > >
> > >
> > >
> > > Hi Authors,
> > >
> > > In
> > > https://tools.ietf.org/html/draft-ietf-pce-segment-routing-policy-cp
> > > -01#section-4.2,  you state -
> > >
> > >    The Association Source MUST be set to the PCC's address.  This
> > >    applies for both PCC-initiated and PCE-initiated candidate paths.
> > >    The reasoning for this is that if different PCEs could set their own
> > >    Association Source, then the candidate paths instantiated by
> > >    different PCEs would by definition be in different PCEP Associations,
> > >    which contradicts our requirement that the SR Policy is represented
> > >    by an Association.
> > >
> > >
> > >
> > >
> > >    The Association ID MUST be chosen by the PCC when the SR policy is
> > >    allocated.  In PCRpt messages from the PCC, the Association ID MUST
> > >    be set to the unique value that was allocated by the PCC at the time
> > >    of policy creation.  In PCInit messages from the PCE, the Association
> > >    ID MUST be set to the reserved value 0, which indicates that the PCE
> > >    is asking the PCC to choose an ID value.  The PCE MUST NOT send the
> > >    Extended Association ID TLV in the PCInit messages.
> > >
> > >
> > > But the base RFC 8697 https://www.rfc-editor.org/rfc/rfc8697.html#section-6.1.3 gave quite a bit of leeway while setting the association source.
> > >
> > > Consider 2 PCEs - PCE1 & PCE2, I am assuming if candidate paths are created via two different PCEs both will be aware of SR Policy identifiers (color, end-point, etc). When PCE1 initiates CP1, it could use the association source as Virtual-IP or NMS (instead of PCE1). The PCE2 will learn about the association and the corresponding SR policy parameters via the PCRpt message which is sent to both PCEs. So when the PCE2 initiates CP2, it could use the same association!
> > >
> > > This was the very reason to include the flexibility in setting the association source in RFC 8697.
> > >
> > > Julien and I discussed this and we feel you are trying to solve the issue of sharing an association ID between several PCEs by using a new mean than the one in RFC 8697. If you have other reasons then please state them, otherwise, RFC 8697 should take precedence.
> > >
> > > Thanks!
> > > Dhruv & Julien
> > >
> > > PS. I quickly drew a figure if that helps (see attached)!
> > >
> > >
> > >
> > > On Tue, Oct 27, 2020 at 8:42 PM <[e-mail]> wrote:
> > >
> > >
> > > A New Internet-Draft is available from the on-line Internet-Drafts directories.
> > > This draft is a work item of the Path Computation Element WG of the IETF.
> > >
> > >         Title           : PCEP extension to support Segment Routing Policy Candidate Paths
> > >         Authors         : Mike Koldychev
> > >                           Siva Sivabalan
> > >                           Colby Barth
> > >                           Shuping Peng
> > >                           Hooman Bidgoli
> > >         Filename        : draft-ietf-pce-segment-routing-policy-cp-01.txt
> > >         Pages           : 20
> > >         Date            : 2020-10-27
> > >
> > > Abstract:
> > >    This document introduces a mechanism to specify a Segment Routing
> > >    (SR) policy, as a collection of SR candidate paths.  An SR policy is
> > >    identified by <headend, color, end-point> tuple.  An SR policy can
> > >    contain one or more candidate paths where each candidate path is
> > >    identified in PCEP via an PLSP-ID.  This document proposes extension
> > >    to PCEP to support association among candidate paths of a given SR
> > >    policy.  The mechanism proposed in this document is applicable to
> > >    both MPLS and IPv6 data planes of SR.
> > >
> > >
> > >
> > > The IETF datatracker status page for this draft is:
> > > https://datatracker.ietf.org/doc/draft-ietf-pce-segment-routing-poli
> > > cy-cp/
> > >
> > > There are also htmlized versions available at:
> > > https://tools.ietf.org/html/draft-ietf-pce-segment-routing-policy-cp
> > > -01
> > > https://datatracker.ietf.org/doc/html/draft-ietf-pce-segment-routing
> > > -policy-cp-01
> > >
> > > A diff from the previous version is available at:
> > > https://www.ietf.org/rfcdiff?url2=draft-ietf-pce-segment-routing-pol
> > > icy-cp-01
> > >
> > >
> > > Please note that it may take a couple of minutes from the time of
> > > submission until the htmlized version and diff are available at tools.ietf.org.
> > >
> > > Internet-Drafts are also available by anonymous FTP at:
> > > ftp://ftp.ietf.org/internet-drafts/
> > >
> > >
> > > _______________________________________________
> > > Pce mailing list
> > > [e-mail]
> > > https://www.ietf.org/mailman/listinfo/pce
> >
> > _______________________________________________
> > Pce mailing list
> > [e-mail]
> > https://www.ietf.org/mailman/listinfo/pce
> _______________________________________________
> Pce mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/pce